Vacuum cleaning is a well-known method for removing dust from surfaces, particularly floors. In general, in the field of vacuum cleaning, a suction force is generated and applied for forcing dust and particles to move from a surface to be cleaned to another location such as a canister for collecting the particles. In the process, it may be desirable to agitate the surface in order to facilitate removal of the particles from the surface under the influence of the suction force as mentioned. To that end, it is possible to use a tool for actually contacting the surface to be cleaned. However, it is also known to use another technique, namely a technique which involves the use of a kind of air pump, wherein air waves are generated for vibrating the surface, which can help in releasing dust particles from the surface.
U.S. Pat. No. 7,383,607 discloses an agitation apparatus which is suitable for use in a cleaning head of a vacuum cleaner, and which includes first and second flow paths. Each of these flow paths has a resonant cavity and an inlet/outlet port which joins the cavity to a space within the cleaning head. A generator, such as a loudspeaker with a diaphragm, generates an alternating pressure wave between the ports. Pressure waves are emitted from one of the ports in an anti-phase relationship with the pressure waves from the other of the ports, thus reducing operating noise. When the vacuum cleaner of which the agitation apparatus is part is used for cleaning a carpet, the air motion to/from the ports vibrates the pile of the carpet and serves to draw out dust from between the carpet fibers.
U.S. Pat. No. 7,383,607 teaches that in the known agitation apparatus, the frequency of the oscillating airflow is preferably chosen such as to be at the resonant frequency of the carpet to be cleaned. Therefore, it is preferred if the frequency of operation is variable.
It is noted that the agitation apparatus known from U.S. Pat. No. 7,383,607 helps in releasing dust from a carpet, but it is not capable of effectively freeing dust from inside a carpet and making it airborne. This cannot be done by only causing a vibration as mentioned, even if a frequency at which the vibration takes place is in the resonant range.